x86/nmi: Perform a safe NMI stack trace on all CPUs

x86/nmi: Perform a safe NMI stack trace on all CPUs

When trigger_all_cpu_backtrace() is called on x86, it will trigger an
NMI on each CPU and call show_regs(). But this can lead to a hard lock
up if the NMI comes in on another printk().

In order to avoid this, when the NMI triggers, it switches the printk
routine for that CPU to call a NMI safe printk function that records the
printk in a per_cpu seq_buf descriptor. After all NMIs have finished
recording its data, the seq_bufs are printed in a safe context.

Link: http://lkml.kernel.org/p/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Tested-by: Jiri Kosina <[email protected]>
Acked-by: Jiri Kosina <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>